The present invention relates to a spark ignition type stratified charge combustion internal combustion engine.
When injecting fuel in a diesel engine, part of the fuel immediately evaporates and forms a preliminary air-fuel mixture while the remaining fuel is made to diffuse in a combustion chamber in the form of fuel droplets. Next, the preliminary air-fuel mixture is made to self-ignite. This forms a heat source for successive combustion of the fuel vaporizing from the fuel droplets. That is, diffusion combustion is performed. In this case, if the ratio of the preliminary air-fuel mixture becomes too great, explosive combustion will occur, so the combustion temperature will sharply rise and therefore a large amount of NOx will be produced.
Therefore, known in the art is a diesel engine designed so that a combustion pressure sensor and spark plug are arranged in the combustion chamber, the ratio of the preliminary air-fuel mixture is found from the detected combustion pressure, when the ratio of the preliminary air-fuel mixture becomes greater than an optimum value, the spark timing is advanced to cause the spark plug to make the preliminary air-fuel mixture ignite before the amount of generation of the preliminary air-fuel mixture increases excessively, and when the ratio of the preliminary air-fuel mixture at the time of ignition by the spark plug becomes smaller than the optimum value, the spark timing is delayed to make the preliminary air-fuel mixture ignite by the spark plug when the amount of generation of the preliminary air-fuel mixture increases to a suitable amount (see Japanese Unexamined Utility Model Publication (Kokai) No. 2-141648).
On the other hand, in a spark ignition type internal combustion engine as well, when the pressure and temperature in the combustion chamber become extremely high, self-ignition occurs. If self-ignition occurs in a spark ignition type internal combustion engine, however, the entire preliminary air-fuel mixture will burn all at once, so the pressure and temperature in the combustion chamber will rapidly rise and as a result not only will knocking occur, but also a large amount of NOx will be produced. Therefore, in the past, spark ignition type internal combustion engines have been designed as much as possible so as not to allow self-ignition and so that the air-fuel mixture is made to burn by the flame of ignition by a spark plug.
If self-ignition occurs, however, as explained above, knocking occurs and a large amount of NOx will be produced. Combustion by self-ignition, however, is completed in a short time. Therefore, if combustion by self-ignition is caused, the thermal efficiency rises. Therefore, if it were possible to cause combustion by self-ignition without causing knocking and the production of a large amount of NOx, a practical internal combustion engine with a high thermal efficiency could be obtained.
Whether or not knocking and production of a large amount of NOx are caused when combustion by self-ignition occurs, however, depends on the amount of the preliminary air-fuel mixture able to be burned by self-ignition and the combustion time. That is, if the entire preliminary air-fuel mixture is burned all at once, as explained above, knocking and production of a large amount of NOx will occur. If however not all of the preliminary air-fuel mixture, but only part of the preliminary air-fuel mixture is made to burn by self-ignition and the preliminary air-fuel mixture is made to burn by self-ignition little by little with a time lag, the pressure and temperature in the combustion chamber will not rise that much and therefore knocking and production of a large amount of NOx will no longer occur. If the amount of the preliminary air-fuel mixture burned by self-ignition in this case is too small, however, no improvement in the thermal efficiency can be expected. Therefore, there is a lower limit to the amount of the preliminary air-fuel mixture which should be made to be burned by self-ignition.
That is, to improve the thermal efficiency while preventing knocking and production of a large amount of NOx, it is necessary to maintain the ratio of the preliminary air-fuel mixture burned by self-ignition in an optimum range and ensure that the preliminary air-fuel mixture burns successively by self-ignition with a time lag.
An object of the present invention is to provide a spark ignition type stratified charge combustion internal combustion engine causing part of the preliminary air-fuel mixture to burn by self-ignition and thereby improving the thermal efficiency, reducing torque fluctuation, and enabling a reduction in the amount of production of unburnt hydrocarbons.
According to the present invention, there is provided a spark ignition type stratified charge combustion internal combustion engine arranging a spark plug in a combustion chamber, said spark ignition type stratified charge combustion internal combustion engine forming by stratification a self-ignitable preliminary air-fuel mixture in the combustion chamber to give a spatial distribution to the density of the preliminary air-fuel mixture in the combustion chamber, igniting part of the preliminary air-fuel mixture formed in the combustion chamber by the spark plug to cause combustion by flame propagation, then successively making the remaining preliminary air-fuel mixture burn by self-ignition with a time lag, and setting the ignition timing so that the ratio of the preliminary air-fuel mixture made to burn by self-ignition becomes more than a predetermined lower limit and less than a knocking generation limit.